1. Field of the Invention
The present invention relates to a sheet feeder employed in an image forming apparatus such as a printer, a copying machine, and a facsimile and particularly relates to a sheet feeder feeding a stack of a plurality of recording media by separating the stacked media one by one.
2. Description of the Related Art
A friction pad separation type sheet feeder has been used in image forming apparatus such as a printer, a copying machine, and a facsimile as a mechanism for feeding stacked recording media or originals by separating each piece at a time. The friction pad separation type sheet feeder feeds recording media by separating the media one by one by a separation nip formed between a paper feeding roller and the friction pad provided to face the paper feeding roller.
In the friction pad separation type sheet feeder, a recording medium passes over the friction pad while being separated to be fed. During the process, a recording medium having a larger width than the friction pad is bent across the width direction to have a convex shape, and both edges of the medium in the width direction droop. When the recording medium passes over the friction pad with both edges drooping, the edges of the recording medium in the width direction will be caught on the edges of the friction pad to cause feeding failure. Moreover, when the recording medium has a bur produced during cutting, the bur digs into one of the ends of the friction pad to cause feeding failure.
To prevent the feeding failure described above, for example, Japanese Patent Application Laid-open No. 06-144619 discloses a sheet feeder in which a recording medium is prevented from being caught on in the following manner. A guiding plate having a low coefficient of friction covers the leading end of the front surface portion of a holder holding a friction pad on a paper feeding side and both edges of the paper feeding surface of the friction pad in the width direction. Japanese Patent Application Laid-open No. 05-278884 also discloses a sheet feeder in which a bur of a recording medium is prevented from digging in an edge of a friction pad by installing a guiding member at both edges of the friction pad and guiding the leading end of the recording medium with the top surface of the guiding member.
Moreover, in the friction pad separation type sheet feeder, the leading end of a recording medium to be fed comes into contact with a friction pad before reaching a separation nip, causing drawback and failing to properly feed the recording medium.
A sheet feeder for dealing with the drawbacks is also disclosed in that a low friction sheet is provided in front of the separation nip formed partly by a friction pad to prevent the friction pad from wearing and to properly feed originals.
However, the sheet feeders disclosed in Japanese Patent Application Laid-open No. 06-144619 and Japanese Patent Application Laid-open No. 05-278884 cause feeding failure in the following manner as illustrated in FIGS. 11 and 12. When a recording medium P is fed into a separation nip formed between a separation roller 102 and a friction pad 103 by being pressed by a pick-up roller 101 to form a downwardly curved concave portion at the middle portion in the width direction, the leading end of the concave portion may hit the friction pad 103 to cause feeding failure. That is, the sheet feeders disclosed in Japanese Patent Application Laid-open No. 06-144619 and Japanese Patent Application Laid-open No. 05-278884 can prevent feeding failure arising from the contact between the edges of the recording medium P in the width direction and the friction pad 103 by using a guiding plate or a guiding member, but cannot prevent the collision between the leading end of the concave portion in the sheet feeding direction and the friction pad 103 and feeding failure that results.
Although a conventional sheet feeder in which a low friction sheet is provided to a friction pad can alleviate the resistance due to the collision by the action of the low friction sheet when the concave portion hits the friction pad as well, the number of components included in the sheet feeder increases due to the inclusion of the low friction sheet. Therefore, the increase in component cost and cost involved in component management and assembly has been unavoidable.